Enhanced cell performance by controlling the surface morphology of ZnO buffer layers in organic photovoltaic cells

Thin layer of ZnO with mixture of spherical and rod-shaped nanoparticles (NM) was prepared as the buffer layer in inverted organic photovoltaic cells (OPVs). The spherical and rod-shaped ZnO nanoparticles were synthesized through sol-gel method to investigate the effect on surface area of ZnO buffer layer. The prepared samples were made up of a 50/50 wt.% mixture of spherical ZnO and rod-shaped ZnO nanoparticles. The surface area of ZnO thin layer prepared with mixture of spherical and rod-shaped nanoparticles has the higher specific surface area than that of ZnO film prepared with spherical or rod-shaped nanoparticles, respectively. The high surface area in buffer layer induced the fast electron transfer in OPVs. Also, the sample NM has the rough surface structure due to the morphology of mixed ZnO nanoparticles. The rough structure increased the contact area at the interface (ZnO buffer layer/active layer). Due to these two reasons, the series resistance (Rs) of the device was decreased. By reduction of the Rs, the current density (Jsc) and fill factor (FF) of device were improved. The highest power conversion efficiency (PCE) with NM is achieved for 2.75% under 100 mW/cm2 at AM 1.5 G simulated solar emission.